Publish-subscribe (pub/sub) is a messaging pattern where devices sending messages, called publishers, do not program the messages to be sent directly to specific receiving devices, called subscribers, but instead categorize published messages into classes without knowledge of which subscribers, if any, there can be. Similarly, subscribers express interest in one or more classes and only receive messages that are of interest, without knowledge of which publishers, if any, are associated with the system. A system using a pub/sub pattern can be referred to as a pub/sub system.